Do we need escape velocity?

[vlad777]

Do we need escape velocity to get back from crossing an event horizon?
Wouldn't a thruster that is outputting more force than weight do?
We would not be in zero G but could get high.

(Like the space jump from that Austrian guy, Felix Baumgartner.
He was not in zero G but he was in space, just because balloon was light enough.)


Many thanks.

[Balaur]

Wouldn't a thruster that is outputting more force than weight do?

Then you have acceleration (or propulsion). If in the right direction, you will move away from Earth.

The escape velocity is computed for objects without propulsion, as a function of the object's kinetic energy when modulo equal (opposite) to the gravitational potential energy.

[John Coloccia]

Do we need escape velocity to get back from crossing an event horizon?
Wouldn't a thruster that is outputting more force than weight do?
We would not be in zero G but could get high.

(Like the space jump from that Austrian guy, Felix Baumgartner.
He was not in zero G but he was in space, just because balloon was light enough.)


Many thanks.

There's no escaping from the even horizon. That is specifically a term attached to black holes. In fact, due to frame dragging, escape velocity is not even enough near the horizon...or said a more precise way, good luck attaining it.

Yes, you can just blast away, but usually when you're talking about spacecraft, you're talking about practical orbits, and just brute force blasting away doesn't get you that. Honestly, I'm not even really sure how important "escape velocity" is as a quantity, other than an interesting thing for your mind to consider. Launching things out of cannons, hoping escape the Earth's clutches, is just not something we ever do. On occasion that we do go off into space, escape velocity is again pretty meaningless since you're specifically looking to solve for an orbit that gets you to some specific location somewhere, not just some academic speed you have to attain just to not fall back down.

Not sure if that answers your question but I tried! 

[vlad777]

I am thinking about a probe reporting back (from inside of a black hole), so I do not need to go in orbit.

[continuo]

The probe, once fallen behind the EH, would need to accelerate to a speed higher than the speed of light, to escape the Black Holes gravitational pull. And that's not possible, regardless of how much thurst you use. At least that's how I understand it.   

[vlad777]

The probe, once fallen behind the EH, would need to accelerate to a speed higher than the speed of light, to escape the Black Holes gravitational pull. And that's not possible, regardless of how much thurst you use. At least that's how I understand it. 

EXACTLY!  That is what we are all told. But if you do not want to "escape the Black Holes gravitational pull", what if you just want to get high?

Of course, I don't understand that little quirk about space falling in faster than speed of light.

[continuo]

Well, you wrote you want the probe to report something back. And that's not possible, you can't get any useful information out of a Black Hole. Maybe someone with more insight will chime in but as far as I know we don't even know if the laws of physics, as we know them, have the slightest meaning after you've crossed the border... 

[vlad777]

Leonard Susskind and Stephen Hawking both said that
if you had the right kind of BH (meaning right size and not active accretion)
you would happily cross the event horizon and not even know it.


So in classical physics terms ,what kind (size) of black hole do we need for the probe weight to be practical?

Edit:
Probe can report everything it seen coming from the outside, and that would be significant information.

[Mechanical Menace]

Leonard Susskind and Stephen Hawking both said that
if you had the right kind of BH (meaning right size and not active accretion)
you would happily cross the event horizon and not even know it.

If a black hole is massive enough yes you could enter the event horizon and not know, depending on which solution to combining quantum and relativistic physics you're using. You wouldn't notice any tidal forces if the event horizon has a large enough diameter so for all intents and purposes you wouldn't notice you'd gone too far. Until you try to retrace your steps.


This of course assumes there are no 'firewalls.'

[vlad777]

Leonard Susskind and Stephen Hawking both said that
if you had the right kind of BH (meaning right size and not active accretion)
you would happily cross the event horizon and not even know it.

If a black hole is massive enough yes you could enter the event horizon and not know, depending on which solution to combining quantum and relativistic physics you're using. You wouldn't notice any tidal forces if the event horizon has a large enough diameter so for all intents and purposes you wouldn't notice you'd gone too far. Until you try to retrace your steps.


This of course assumes there are no 'firewalls.'

What would be your weight?

[SeanB]

You do not notice your crossing the event horizon, it is just that at that point that to get a signal out your gamma ray signal generator ( about the highest frequency you can generate that still will radiate as a photon) will be red shifted so much that the receiving end will be unable to receive it. In fact a while before the photons you emit back from the probe will be red shifted to close to DC, and will be non receivable at the other non near station in any form. Even launching a small probe back will not work, it will have to leave at close to C. Think of the problem of launching something physical at close to C when doing so will need the entire energy output of a large sun for a decade, to do so for anything over a few hundred thousand atoms, even for a dust mote. Past the event horizon your photons leave you at C going out ( relative to you), but your local time is distorted so much by the local gravity field that they go out but will still be travelling into the singularity at a velocity that from the outside is greater than C.

[vlad777]

"it will have to leave at close to C"

This is what I am not understanding/ questioning.

C is the escape velocity at EH, but we do not need to escape (just move a little and hover).

[continuo]

C is the escape velocity at EH, but we do not need to escape (just move a little and hover).

Of course you have to escape if you want to report something back. It doesn't matter if it's the probe itself or some sort of communication signal. It has to escape the EH to be recognised by an outside observer. Or maybe I don't understand your question 

[IanB]

Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

[Chris C]

Leonard Susskind and Stephen Hawking both said that
if you had the right kind of BH (meaning right size and not active accretion)
you would happily cross the event horizon and not even know it.

I'm not familiar with that statement by Susskind and Hawking, but I'm pretty sure you've misinterpreted it.  If "cross" means only passing near the black hole, and if it's small and not actively accreting, then that statement is correct; you may not realize you've passed it.

But you cannot actually cross through (in and out) of the event horizon.  The event horizon is the distance at which the gravitational pull accelerates things towards the black hole at the speed of light.  Producing an opposing equal acceleration to hold position is impractical, and a greater acceleration to escape is impossible.

So in classical physics terms ,what kind (size) of black hole do we need for the probe weight to be practical?

Subatomic.  Anything larger would be shredded by gravitational shear forces before reaching the event horizon.

Even if that wasn't the case, a probe in the traditional sense would be useless, due to temporal effects near strong gravity wells.   What kind of useful information can you transmit when the probe's 3 gigahertz processor, in its brief descent into expungement, looks to the rest of the universe like it's operating at 1 femtohertz?

But quantum entangled subatomic particles may be able to transmit some sort of information out.

[miguelvp]

Is this in reference to Stephen Hawkin's new black hole theory that he just recently published? (Edit: not sure about publishing but he at least announced it).

I have not had a chance to read anything about it, but plenty to find with google.

[vlad777]

Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

(You are in a stable orbit above EH and have a probe on a tether, that could be equivalent.)

Yes ,tell me please.

Edit:
Ok the tether breaks.

[vlad777]

Is this in reference to Stephen Hawkin's new black hole theory that he just recently published? (Edit: not sure about publishing but he at least announced it).

I have not had a chance to read anything about it, but plenty to find with google.

No.

[John Coloccia]

Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

Let me put it this way. In YOUR time frame, you would never actually see the probe ever reach the event horizon. It would simply gradually fade, getting dimmer and dimmer...forever. To outside observers, time grinds to a halt at the horizon. The early name of black holes was "frozen star", because when it eventually collapsed, it would appear to freeze right at the event horizon.

The first step in starting to understand what really happens here is to start understanding that gravity works as much because TIME warps as it does because SPACE warps. The event horizon is that special place where time simply stops from the reference frame of an outside observer. Very strange things happen, but it's also a beautiful result when you consider it hand in hand with some of the equivalencies between thermodynamics and black holes, information theory, etc etc. It seems very fitting that everything should be forever frozen on the surface of the event horizon.

[vlad777]

Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

Let me put it this way. In YOUR time frame, you would never actually see the probe ever reach the event horizon. It would simply gradually fade, getting dimmer and dimmer...forever. To outside observers, time grinds to a halt at the horizon. The early name of black holes was "frozen star", because when it eventually collapsed, it would appear to freeze right at the event horizon.

So even if it could come back, it would be MUCH later, thanks I think I get this part.

[vlad777]

Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

Tether is a deal breaker here, my probe has a thruster which makes it independent of the outside space-time.
Which means it can pop up anywhere anytime.

[vlad777]

OK I am a layman, but I am referring to the idea that has been drilled to our minds.
The idea that something needs escape velocity to get out of a black hole, so since this is C, exiting a black hole is impossible.
But you do not need orbital or escape velocity to exit a region.
You need force. As long as this force is equal or grater than your weight you will ascend.

[IanB]

You need force. As long as this force is equal or grater than your weight you will ascend.

That's the point of the tether. The tether can apply an upward force to counteract the downward gravitational force.

But the point of escape velocity is that it corresponds to the amount of energy required to lift a mass M out of a gravitational well. For instance if you wanted to remove a mass M from the Earth's surface to a distance far from Earth's gravitational field the energy required would be found by integrating force over distance, i.e. integrate the gravitational force on M from zero to infinity. Since gravitational force obeys an inverse square law this amount of energy will be finite.

Now the kinetic energy of a mass M is given by ½Mv². If you equate the two energies and solve for v you get the initial speed you would have to give the mass to reach an infinite distance from Earth. This is the escape velocity. If you solve for v and v is greater than the speed of light, then you discover that you cannot escape the gravitational well. It would require infinite energy to do so, and that is physically unfeasible. Your thruster could never carry enough fuel.

[John Coloccia]

OK I am a layman, but I am referring to the idea that has been drilled to our minds.
The idea that something needs escape velocity to get out of a black hole, so since this is C, exiting a black hole is impossible.
But you do not need orbital or escape velocity to exit a region.
You need force. As long as this force is equal or grater than your weight you will ascend.

Just because there are no drastic TIDAL forces at a supermassive black hole's event horizon doesn't mean there is no force. It's still an even horizon. It gets to be one because gravity is so strong that nothing can escape. In free fall, you feel nothing because you're in free fall (i.e. you're in an inertial reference frame). Try to fire some rockets and you will quickly find that there is no rocket powerful enough to resist the pull of gravity. Said another way, all space-time paths in a black hole lead to the singularity...period. There's no trick to get around it. If you're inside the event horizon, you're going to the singularity, do not pass go, do not hesitate, do not wander about. That's where you're heading.

Tether does you no good. If you pull hard enough, the tether will eventually just break without making the slightest bit of progress to drag the other thing back out. If the tether doesn't break, it will simply drag in anything else attached to it.

Ian's explanation is good because it touches on the idea of a required ENERGY to escape the gravitational well. Thrusters, tethers, escape velocity or not, it's the energy required that dooms you to a steady march into the singularity.

[vlad777]

" Try to fire some rockets and you will quickly find that there is no rocket powerful enough to resist the pull of gravity."

Does this mean that their weight is infinite?

Edit:

Do you mean they will not move at all, or do you mean that they will eventually come back?